1. Field of the Invention
The present invention relates to a wireless communication system used for Multi-Carrier CDMA (Code Division Multiple Access) with Code Selection method or Multi-Code CDMA with Code Selection method.
2. Description of the Related Art
It is generally understood that the orthogonal frequency division multiplexing method (OFDM) will be the next-generation fast wireless communication method. Multicode techniques or multicarrier techniques are applied to implement the fast wireless communication. However, it is pointed out that use of the multicode techniques or the multicarrier techniques makes a communication speed fast, but produces a number of problems such as a large PAPR (peak to average power ratio) or complicated signal processing for wireless communication. In particular, it is necessary to use a high-performance radio frequency (RF) circuit (a linear amplifier) when the PAPR is large.
Precisely speaking, Multi-code code division multiple access (MC-CDMA) system is known to have more capacity than the single code CDMA system so that it can transmit high speed data rates for various applications. However, the transmitting signal of MC CDMA has a high peak to average power ratio (PAPR), and a highly linear power amplifier may be required for transmission. This problem is the same to the OFDM (orthogonal frequency division multiplexing) system. So, to transmit the high PAPR signal without distortion requires more expensive power amplifier with high linearity and wider dynamic range. The impact of amplifier nonlinearity is investigated since the efficiency if high power amplifier (HPA) depends on the PAPR. Consequently, the purpose for designing a signal with low PAPR is an important problem to decrease the HPA nonlinearity and to improve the power efficiency of HPA.
The binary code division multiple access (CDMA) method configured to adjust the PAPR within a predetermined range has been proposed to deal with these problems (see Patent Document 1 (S. M. Ryu, “Code Select CDMA Modulation and Demodulation Method and Device Thereof,” Korea Patent, no. 10-2001-006178, October 2001), for example). According to the modulation method used therein, this binary CDMA method can be further divided into categories of the pulse width CDMA method (hereinafter referred to as the PW/CDMA method), the multi-phase CDMA method (hereinafter referred to as the MP/CDMA method), and the code select CDMA method (hereinafter referred to as the CS/CDMA method).
The CS/CDMA method is the method configured to treat inputted data in units of multiple blocks each having a predetermined number of bits and to assign one orthogonal code to each block unit as a channel code. For this reason, unlike the PW/CDMA method or the MP/CDMA method, a phenomenon of performance deterioration attributable to mutual interference between terminals such as incapability of receiving data normally is not observed in this method.
In this CS/CDMA method, it is extremely important to reduce errors in selection of orthogonal codes at the time of data reception. This is because the selecting of one orthogonal code for multiple bits results in errors for the multiple bits, if the selected orthogonal code is an error.
To solve this serious problem, several schemes have been proposed where the PAPR of MC CDMA can be reduced with special coding techniques. In above Patent Document 1 (S. M. Ryu, “Code Select CDMA Modulation and Demodulation Method and Device Thereof,” Korea Patent, no. 10-2001-006178, October 2001), a new CDMA concept called Binary CDMA has been proposed to obtain constant amplitude signal. Binary CDMA is classified into Pulse Width (PW) CDMA where the multilevel signals are converted into pulse width, Multi Phase (MP) CDMA where the multilevel signals are converted into phases, and Code Select (CS) CDMA where the multilevel signals have multi-code characteristics by selecting a code without truncation. The advantage of these schemes is that PAPR is reduced since fewer codes are used in the linear sum compared with the conventional multi-code CDMA. However, the orthogonal characteristics of the signal are weakened by truncation, and there causes a performance degradation in PW CDMA system and MP CDMA system. Also, performance of CS CDMA system is depended on detection error due to signal distortion. Therefore, the CS CDMA system which is robust to signal distortion must be considered.